Artificial radioactive isotopes of a natural element can be used to visualize and treat specific cancer cells in the body. All that is needed is to administer tiny amounts of a so-called radiopharmaceutical to the patient. Thanks to specific receptors, the radiopharmaceuticals accumulate only in the tumor but not in healthy tissue. A prominent example is the use of radioactive iodine to diagnose and treat thyroid cancer. Recently, new radiopharmaceuticals for the detection and treatment of prostate cancer and neuroendocrine tumors have been successfully tested in the clinic. In this lecture, we will take you along the development path of a radiopharmaceutical from the laboratory to the clinic.